The present invention discloses a method of plating aluminum nitride with electroless plating solutions while protecting the aluminum nitride from the electroless solution.
Materials with high thermal conductivity and low dielectric constant are required for high speed and high power microwave hybrid circuits. Aluminum Nitride (AlN) meets these standards and provides additional benefits as well. Alumina and beryllia are typical substrates used in microwave applications but are gradually being replaced by other substrates. One of the major concerns in the processing of circuits on AlN, is AlN's extreme susceptibility to alkaline solutions. This presents a problem not only while cleaning the substrate but also during various metal etching and plating processes.
Aluminum nitride is an exceptionally difficult material to plate using electroless techniques. Most electroless gold plating solutions have a very high pH, usually 14, and aluminum nitride is very susceptible to solutions with even a mildly high pH. It has been observed that even deionized water, with an average pH of 8.2, can have detrimental effects on the aluminum nitride. The effect of an alkaline solution on aluminum nitride is an actual etching of the surface. Exposed to a detergent solution for only seconds, the surface of AlN may be altered enough to substantially reduce the adhesion of thin and thick film metals.
Aluminum nitride substrates were placed in sodium hydroxide solutions having a pH of 14 and the etch rate was calculated. The control, aluminum oxide, had an etch rate of 0.00% while that of aluminum nitride had an etch rate of between 3.16 and 4.10% by weight per hour depending on the concentration of the solution. This is an unacceptable rate of deterioration for any circuit application and will inevitably cause severe problems in future processing of the circuit.
Problems which arise because of this substrate etching effect are three fold. First, the plated pattern deposited by the electroless solution will have very poor adhesion, often to the extreme that the edges of pad areas and transmission lines will peel off during rinsing. Second, the solution may readily become contaminated by the disassociated AlN. Electroless plating solutions are extremely sensitive to all types of contamination. A buildup of aluminum in the plating bath will very quickly inactivate the solution as well as contaminate substrates being plated. The third problem is that of determining the thickness of the plated films. Typically, the thickness of plated metals is determined by a weight gain calculation. A substrate is weighed before and after plating and a calculation involving the density of plated gold can yield a very accurate value for gold thickness. This method, however, is inconclusive using aluminum nitride. When plating electroless gold onto a metal circuit pattern on aluminum nitride, the etch rate of the substrate far exceeds the deposition rate of the gold. The net effect is a decrease in overall weight of the substrate. Another means of measuring thickness is a stylus profilometer. In this process, a stylus is dragged across the surface of the substrate and differences in height are physically measured. For aluminum nitride, a stylus profilometer cannot be used to measure the thickness step since the measurement will be taken from the bottom of the etched substrate to the surface of the plated gold on the unetched areas, therefore, showing an artificially high thickness of gold. Because of the variations in density for plated gold, x-ray fluorescence is also not an accurate means of determining film thickness.
The etching of aluminum nitride is only a problem during plating when a conductor pattern has previously been etched, thereby leaving exposed ceramic material (i.e. AlN) in the plating solution. One option, which is not always available, is to plate up gold onto a completely metallized substrate and then pattern and etch the circuit. This process does not allow for very fine line definition or control of side wall profiles compared to pre-patterned plating. In addition, this method is very costly and inefficient.
The present invention overcomes these obstacles in a novel manner and thereby allows the use of aluminum nitride as a substrate for microwave circuits.